The present invention relates to adhesives adapted to bond optical elements and more particularly color mosaic filters over solid-state image sensors.
In order to cement optical lens elements to each other, two types of adhesives are used. One type is thermosetting adhesives such as balsam (the product of E. MERCK A.G.), epoxy adhesives, polyester adhesives, adhesives consisting of polyvinyl acetate, acryl sulfoamide and products of condensation of formalin and the like. The other type is photosetting or photo-curing type adhesives such as polyester adhesives (UV-69, 71, 74 and so on, the products of Summers Laboratories-Inc. USA), thiolic adhesives (NOA-61, 63, 65 and so on, the products of Norland), acrylic-epoxy adhesives and the like.
However, there has not been commercially available an adhesive having optimum properties such as curing characteristics and the like for bonding color mosaic filters to solid-state image sensors. In this regard, bonding of color mosaic filters to solid-state image sensors has many problems as will be described below.
It has been considered that the solid-state color image sensor will make considerable contributions to the reduction both in size and weight; that is, to the miniaturization of video cameras. When it is desired to use only one solid-state image sensor in each color camera, a three-color filter (comprising, for instance, red, green and cyane filter elements) must be bonded to the front surface of an array picture elements. At present organic filter elements; that is, three-color filter elements made of organic compounds are widely used and deposited or otherwise formed in a predetermined mosaic pattern over an optical glass sheet. A filter glass sheet thus obtained is bonded over a solid-state image sensor.
When thermosetting type resins which have been widely used in cementing optical lens elements are used, a higher degree of bond strength can be attained. However, the recent trend of the video camera is toward making it more compact in size and lighter in weight so that much efforts have been made in reducing the area of each of picture elements of the solid-state image sensor. In addition, in order to obtain a higher degree of resolution and also a higher degree of optical sensitivity, the spacing between the adjacent picture elements must be reduced as much as possible. As a consequence, various processes have been devised and demonstrated for registering a filter glass sheet with a solid-state image sensor in such a way that every color filter elements on the color filter glass sheet can be correctly aligned with every corresponding picture element on the solid-state image sensor in a one-to-one relationship with a higher degree of dimensional accuracy. However, it has been recently found that with the prior art thermosetting resins, exact registration cannot be attained. More specifically, when a color filter glass sheet and a solid-state image sensor, each having a bonding or contact area of 200 mm.sup.2, are bonded together with a layer of an adhesive of less than 10 .mu.m in thickness, the alignment accuracy; that is, a measurement for alignment between the pattern of picture elements on the solid-state image sensor and the mosaic pattern of color filter elements on the filter glass sheet, must be less than 2 .mu.m. To this end, the step for pressing the color filter glass sheet against the image sensor and the step for bonding them together must be carried out simultaneously and the filter sheet must be kept pressed against the image sensor even after the alignment between them has been accomplished until the adhesive is cured.
However, if the thickness of a layer of a thermocuring resin adhesive between the color filter glass sheet and the solid-state image sensor is less than 10 .mu.m during the alignment or registration step, there arise various problems as described below. First, if the curing time is shorter than the alignment time, curing of the adhesive proceeds gradually so that accurate alignment cannot be attained. If the curing time is considerably longer than the alignment time, the residence time will be increased so that a higher degree of productivity cannot be attained. In addition, the color filter glass sheet and the solid-state image sensor tend to be misaligned during the curing process. The misalignment between the filter and picture elements results in degradation in the signal-to-noise ratio (S/N) of the color signal generated. In addition, flicker and color fringing are observed.
The above problems can be solved if an adhesive used will not be cured during the alignment step, but will be cured as fast as possible after the alignment step. Ultraviolet ray-curing type adhesives have such properties as described above, but the objects of the present invention cannot be accomplished by the mere use thereof.